There have recently been proposals to allow access to the features and services provided by GSM (Global System for Mobile communications) and UMTS (Universal Mobile Telecommunication Services) networks other than by accessing those networks in the conventional manner. In this regard, the conventional manner involves signalling between the mobile terminal and a standard base station (macro base station) that has a dedicated backhaul connection to an MSC (Mobile Switching Centre), and provides coverage in the cell occupied by the mobile terminal using cellular telecommunication (e.g. GSM or UMTS) transport protocols.
To increase network capacity, it has been proposed to provide additional special base stations, often referred to as femto cells, femto base stations, pico cells, pico base stations or access points (APs). These special base stations may be a dedicated network access point, or may be enhanced wireless internet hubs (i.e. providing wireless internet access, as well as wireless telecommunications network access). The range of special base stations is significantly smaller than macro base stations, typically only providing coverage of the order of 20 to 30 meters, making them suitable for use, for example at a subscriber's home or office.
An advantage of using an Access Point connected to the core network via an IP network is that existing broadband DSL connections, or any other IP connection, can be used to link mobile terminals with the network core without using the capacity of the radio access network or transmission network of a mobile telecommunications network. In this regard, these access points communicate with the core network via IP based communications, such as a (fixed) broadband IP network, and are typically routed via the Internet. Using such base stations will allow a proportion of the data required for the provision of features or services to be carried through a fixed network and onto the backhaul network without requiring Macro radio network capacity.
They are also able to provide mobile network access where there is no conventional radio access network coverage. For example, UMTS coverage could be provided by an access point where there is no conventional UMTS coverage.
It has also been proposed to use these APs in the Long Term Evolution (LTE) telecommunications network which is currently being developed. An industry body, 3GPP (Third Generation Partnership Project) is overseeing the production of technical specifications for implementing this next generation mobile system, which is based on an evolved GSM core network. LTE is likely to be the next network implementation after the current 3G UMTS etc.
In future networks, there is likely to be a heterogeneous mixture of Access Points covering a single building, micro base stations serving very small areas and macro base stations serving significantly larger areas. These Access Points will coexist with the conventional base stations with the coverage area of one macro base station likely to overlap with a number of Access Points. Overall, the use of APs as an additional or alternative means for accessing the network will advantageously increase the network capacity and coverage.
In a conventional mobile telecommunications network the number of macro base stations provided will typically be of the order of 10,000. The location, orientation and power of each macro station is carefully planned by a laborious manual process requiring skilled personnel (a cellular network planning specialist). Typically, to correctly set up each macro base station takes perhaps one to three man days. Whilst this is acceptable for a mobile telecommunications network comprising only macro base stations, where the total number of base stations is limited, this is not satisfactory for a mobile telecommunications network comprising a large number of access points. There may be ten or a hundred times as many access points as macro base stations in the mobile telecommunications network in the future. Requiring skilled personnel to perform a laborious set up procedure for each access point is not feasible.
Arrangements for automatically configuring access points are known. One such procedure is described in GB-A-2430839. In such an arrangement, after authentication of an access point, the access point monitors each of the UMTS carriers and scrambling codes provided in a permitted list in turn, and measures the carrier to interference ratio (C/I ratio). The access point then selects the UMTS carrier and scrambling code within the permitted list with minimum received power from surrounding base stations on the basis that these carriers will cause minimum interference to surrounding base stations. The access point then selects initial power levels for the UMTS transmit paths. An appropriate initial power level is deduced from the received signal strength detected by the access point. The goal is that the transmitted power level is sufficient to provide a cellular service at a distance of 20 meters with the level of in-band interference caused by surrounding base stations. Transmit power may be modified during a call to maintain acceptable quality of service (QoS) in accordance with the UMTS standards. Such an access point may, when performing measurements of surrounding base stations, detect both macro base stations and other access points.
Mobile networks such as 2G (GSM), 3G (UMTS) or future LTE telecommunications networks have an active state of communication with their mobile terminals and an inactive/idle state of communication with their terminals. When in the active state, as the mobile terminals move between different cells of the network, the communication session is maintained by performing a “handover” operation between the cells. In the inactive/idle state, as a mobile terminal moves between different cells of the network the mobile terminal performs “cell reselection” to select the most appropriate cell on which to “camp” in order that the mobile terminal can be paged by the network when mobile terminating data is destined for that mobile terminal.
The mobile terminal or network determines whether a handover/cell reselection procedure should be triggered in dependence upon measurements of the radio signals of the cells in the region of the mobile terminal. A filter is applied to the signals (either by the network or by the mobile terminal) which calculates an average (mean) value of these signals over a particular time period. This filtered/average values of the cells are then compared with each other or with a threshold value. In dependence upon these comparisons, cell reselection/handover related procedures are triggered. This cell reselection/handover process generally comprises taking radio signal measurements of neighbouring cells and comparing these to each other and to the radio signal of the current cell to determine which cell provides the best signal strength/quality. Handover/reselection to the best cell can then occur.
It is proposed that, in a cellular network including both access points and conventional macro base stations, a mobile terminal will be able to move between access points and macro base stations in order to provide the best radio coverage in the circumstances. Reselection between an access point and a macro base station, and vice versa, is possible, as is handover from a macro base station to an access point, and vice versa.
The present inventors have identified that in some circumstances it may be desirable to cover a particular area with a plurality of access points—for example, to provide radio coverage in an office that is of a size such that a single access point would be insufficient. The present inventors have further identified that problems can arise when a plurality of access points are activated in the same area and attempt to automatically configure themselves. During this automatic configuration process, each access point will take into account the radio signals transmitted by the other access points. The automatic procedure can cause the access points to “compete” or “race” with each other to establish an appropriate initial power level. The configuration process may take a large amount of time, or may never be completed, as the parameters measured by each access point will vary as the neighbouring access points vary their transmit powers in their attempts to configure themselves.
Embodiments of the system described herein seek to provide an improved arrangement for providing radio coverage in an area served by a plurality of access points.